The present invention is directed to organic light emitting devices (OLEDs) comprised of emissive layers that contain an organometallic phosphorescent compound.
Organic light emitting devices (OLEDs) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device, C. W. Tang et al., Appl. Phys. Lett. 1987, 51, 913. Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, Feb. 1995). Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor, International Patent Application No. PCT/US95/15790.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in International Patent Application Ser. No. PCT/US97/02681 in which the TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mgxe2x80x94Agxe2x80x94ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mgxe2x80x94Agxe2x80x94ITO electrode layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color. This colored emission could be transmitted through the adjacently stacked, transparent, independently addressable, organic layer or layers, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
The PCT/US95/15790 application disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. The PCT/US95/15790 application, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
Because light is generated in organic materials from the decay of molecular excited states or excitons, understanding their properties and interactions is crucial to the design of efficient light emitting devices currently of significant interest due to their potential uses in displays, lasers, and other illumination applications. For example, if the symmetry of an exciton is different from that of the ground state, then the radiative relaxation of the exciton is disallowed and luminescence will be slow and inefficient. Because the ground state is usually anti-symmetric under exchange of spins of electrons comprising the exciton, the decay of a symmetric exciton breaks symmetry. Such excitons are known as triplets, the term reflecting the degeneracy of the state. For every three triplet excitons that are formed by electrical excitation in an OLED, only one symmetric state (or singlet) exciton is created. (M. A. Baldo, D. F. O""Brien, M. E. Thompson and S. R. Forrest, Very high-efficiency green organic light-emitting devices based on electrophosphorescence, Applied Physics Letters, 1999, 75, 4-6.) Luminescence from a symmetry-disallowed process is known as phosphorescence. Characteristically, phosphorescence may persist for up to several seconds after excitation due to the low probability of the transition. In contrast, fluorescence originates in the rapid decay of a singlet exciton. Since this process occurs between states of like symmetry, it may be very efficient.
Many organic materials exhibit fluorescence from singlet excitons. However, only a very few have been identified which are also capable of efficient room temperature phosphorescence from triplets. Thus, in most fluorescent dyes, the energy contained in the triplet states is wasted. However, if the triplet excited state is perturbed, for example, through spin-orbit coupling (typically introduced by the presence of a heavy metal atom), then efficient phosphoresence is more likely. In this case, the triplet exciton assumes some singlet character and it has a higher probability of radiative decay to the ground state. Indeed, phosphorescent dyes with these properties have demonstrated high efficiency electroluminescence.
Only a few organic materials have been identified which show efficient room temperature phosphorescence from triplets. In contrast, many fluorescent dyes are known (C. H. Chen, J. Shi, and C. W. Tang, xe2x80x9cRecent developments in molecular organic electroluminescent materials,xe2x80x9d Macromolecular Symposia, 1997, 125, 1-48; U. Brackmann, Lambdachrome Laser Dyes (Lambda Physik, Gottingen, 1997)) and fluorescent efficiencies in solution approaching 100% are not uncommon. (C. H. Chen, 1997, op. cit.) Fluorescence is also not affected by triplet-triplet annihilation, which degrades phosphorescent emission at high excitation densities. (M. A. Baldo, et al., xe2x80x9cHigh efficiency phosphorescent emission from organic electroluminescent devices,xe2x80x9d Nature, 1998, 395, 151-154; M. A. Baldo, M. E. Thompson, and S. R. Forrest, xe2x80x9cAn analytic model of triplet-triplet annihilation in electrophosphorescent devices,xe2x80x9d 1999). Consequently, fluorescent materials are suited to many electroluminescent applications, particularly passive matrix displays.
To understand the different embodiments of this invention, it is useful to discuss the underlying mechanistic theory of energy transfer. There are two mechanisms commonly discussed for the transfer of energy to an acceptor molecule. In the first mechanism of Dexter transport (D. L. Dexter, xe2x80x9cA theory of sensitized luminescence in solids,xe2x80x9d J. Chem. Phys., 1953, 21, 836-850), the exciton may hop directly from one molecule to the next. This is a short-range process dependent on the overlap of molecular orbitals of neighboring molecules. It also preserves the symmetry of the donor and acceptor pair (E. Wigner and E. W. Wittmer, Uber die Struktur der zweiatomigen Molekelspektren nach der Quantenmechanik, Zeitschrift fur Physik, 1928, 51, 859-886; M. Klessinger and J. Michl, Excited states and photochemistry of organic molecules (VCH Publishers, New York, 1995)). Thus, the energy transfer of Eq. (1) is not possible via Dexter mechanism. In the second mechanism of Fxc3x6rster transfer (T. Fxc3x6rster, Zwischenmolekulare Energiewanderung and Fluoreszenz, Annalen der Physik, 1948, 2, 55-75; T. Fxc3x6rster, Fluoreszenz organischer Verbindugen (Vandenhoek and Ruprecht, Gottinghen, 1951 ), the energy transfer of Eq. (1) is possible. In Fxc3x6rster transfer, similar to a transmitter and an antenna, dipoles on the donor and acceptor molecules couple and energy may be transferred. Dipoles are generated from allowed transitions in both donor and acceptor molecules. This typically restricts the Forster mechanism to transfers between singlet states.
Nevertheless, as long as the phosphor can emit light due to some perturbation of the state such as due to spin-orbit coupling introduced by a heavy metal atom, it may participate as the donor in Fxc3x6rster transfer. The efficiency of the process is determined by the luminescent efficiency of the phosphor (F Wilkinson, in Advances in Photochemistry (eds. W. A. Noyes, G. Hammond, and J. N. Pitts), pp. 241-268, John Wiley and Sons, New York, 1964), i.e., if a radiative transition is more probable than a non-radiative decay, then energy transfer will be efficient. Such triplet-singlet transfers were predicted by Fxc3x6rster (T. Fxc3x6rster,xe2x80x9cTransfer mechanisms of electronic excitation,xe2x80x9d Discussions of the Faraday Society, 1959, 27, 7-17) and confirmed by Ermolaev and Sveshnikova (V. L. Ermolaev and E. B. Sveshnikova, xe2x80x9cInductive-resonance transfer of energy from aromatic molecules in the triplet state,xe2x80x9d Doklady Akademii Nauk SSSR, 1963, 149, 1295-1298), who detected the energy transfer using a range of phosphorescent donors and fluorescent acceptors in rigid media at 77K or 90K. Large transfer distances are observed; for example, with triphenylamine as the donor and chrysoidine as the acceptor, the interaction range is 52 xc3x85.
The remaining condition for Fxc3x6rster transfer is that the absorption spectrum should overlap the emission spectrum of the donor assuming the energy levels between the excited and ground state molecular pair are in resonance. In one example of this application, we use the green phosphor fac tris(2-phenylpyridine) iridium (Ir(ppy)3; M. A. Baldo, et al., Appl. Phys. Lett., 1999, 75, 4-6) and the red fluorescent dye [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl) ethenyl]-4H-pyran-ylidene] propane-dinitrile] (xe2x80x9cDCM2xe2x80x9d; C. W. Tang, S. A. VanSlyke, and C. H. Chen, xe2x80x9cElectroluminescence of doped organic films,xe2x80x9d J. Appl. Phys., 1989, 65, 3610-3616). DCM2 absorbs in the green, and, depending on the local polarization field (V. Bulovic, et al., xe2x80x9cBright, saturated, red-to-yellow organic light-emitting devices based on polarization-induced spectral shifts,xe2x80x9d Chem. Phys. Lett., 1998, 287, 455-460), it emits at wavelengths between xcex=570 nm and xcex=650 nm.
It is possible to implement Fxc3x6rster energy transfer from a triplet state by doping a fluorescent guest into a phosphorescent host material. Unfortunately, such systems are affected by competitive energy transfer mechanisms that degrade the overall efficiency. In particular, the close proximity of the host and guest increase the likelihood of Dexter transfer between the host to the guest triplets. Once excitons reach the guest triplet state, they are effectively lost since these fluorescent dyes typically exhibit extremely inefficient phosphorescence.
To maximize the transfer of host triplets to fluorescent dye singlets, it is desirable to maximize Dexter transfer into the triplet state of the phosphor while also minimizing transfer into the triplet state of the fluorescent dye. Since the Dexter mechanism transfers energy between neighboring molecules, reducing the concentration of the fluorescent dye decreases the probability of triplet-triplet transfer to the dye. On the other hand, long range Fxc3x6rster transfer to the singlet state is unaffected. In contrast, transfer into the triplet state of the phosphor is necessary to harness host triplets, and may be improved by increasing the concentration of the phosphor.
Devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, OLEDs are comprised of at least two thin organic layers separating the anode and cathode of the device. The material of one of these layers is specifically chosen based on the material""s ability to transport holes, a xe2x80x9chole transporting layerxe2x80x9d (HTL), and the material of the other layer is specifically selected according to its ability to transport electrons, an xe2x80x9celectron transporting layerxe2x80x9d (ETL). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the hole transporting layer, while the cathode injects electrons into the electron transporting layer. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. Recombination of this short-lived state may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism. Under this view of the mechanism of operation of typical thin-layer organic devices, the electroluminescent layer comprises a luminescence zone receiving mobile charge carriers (electrons and holes) from each electrode.
As noted above, light emission from OLEDs is typically via fluorescence or phosphorescence. There are issues with the use of phosphorescence. It has been noted that phosphorescent efficiency decreases rapidly at high current densities. It may be that long phosphorescent lifetimes cause saturation of emissive sites, and triplet-triplet annihilation may produce efficiency losses. Another difference between fluorescence and phosphorescence is that energy transfer of triplets from a conductive host to a luminescent guest molecule is typically slower than that of singlets; the long range dipole-dipole coupling (Fxc3x6rster transfer) which dominates energy transfer of singlets is (theoretically) forbidden for triplets by the principle of spin symmetry conservation. Thus, for triplets, energy transfer typically occurs by diffusion of excitons to neighboring molecules (Dexter transfer); significant overlap of donor and acceptor excitonic wavefunctions is critical to energy transfer. Another issue is that triplet diffusion lengths are typically long (e.g.,  greater than 1400 xc3x85) compared with typical singlet diffusion lengths of about 200 xc3x85. Thus, if phosphorescent devices are to achieve their potential, device structures need to be optimized for triplet properties. In this invention, we exploit the property of long triplet diffusion lengths to improve external quantum efficiency.
Successful utilization of phosphorescence holds enormous promise for organic electroluminescent devices. For example, an advantage of phosphorescence is that all excitons (formed by the recombination of holes and electrons in an EL), which are (in part) triplet-based in phosphorescent devices, may participate in energy transfer and luminescence in certain electroluminescent materials. In contrast, only a small percentage of excitons in fluorescent devices, which are singlet-based, result in fluorescent luminescence.
An alternative is to use phosphorescence processes to improve the efficiency of fluorescence processes. Fluorescence is in principle 75% less efficient due to the three times higher number of symmetric excited states.
Because one typically has at least one electron transporting layer and at least one hole transporting layer, one has layers of different materials, forming a heterostructure. The materials that produce the electroluminescent emission are frequently the same materials that function either as the electron transporting layer or as the hole transporting layer. Such devices in which the electron transporting layer or the hole transporting layer also functions as the emissive layer are referred to as having a single heterostructure. Alternatively, the electroluminescent material may be present in a separate emissive layer between the hole transporting layer and the electron transporting layer in what is referred to as a double heterostructure. The separate emissive layer may contain the emissive molecule doped into a host or the emissive layer may consist essentially of the emissive molecule.
That is, in addition to emissive materials that are present as the predominant component in the charge carrier layer, that is, either in the hole transporting layer or in the electron transporting layer, and that function both as the charge carrier material as well as the emissive material, the emissive material may be present in relatively low concentrations as a dopant in the charge carrier layer. Whenever a dopant is present, the predominant material in the charge carrier layer may be referred to as a host compound or as a receiving compound. Materials that are present as host and dopant are selected so as to have a high level of energy transfer from the host to the dopant material. In addition, these materials need to be capable of producing acceptable electrical properties for the OLED. Furthermore, such host and dopant materials are preferably capable of being incorporated into the OLED using starting materials that can be readily incorporated into the OLED by using convenient fabrication techniques, in particular, by using vacuum-deposition techniques.
The exciton blocking layer used in the devices of the present invention (and previously disclosed in U.S. appl. Ser. No. 09/153,144, now U.S. Pat. No. 6,097,147) substantialIy blocks the diffusion of excitons, thus substantially keeping the excitons within the emission layer to enhance device efficiency. The material of blocking layer of the present invention is characterized by an energy difference (xe2x80x9cband gapxe2x80x9d) between its lowest unoccupied molecular orbital (LUMO) and its highest occupied molecular orbital (HOMO). In accordance with the present invention, this band gap substantially prevents the diffusion of excitons through the blocking layer, yet has only a minimal effect on the turn-on voltage of a completed electroluminescent device. The band gap is thus preferably greater than the energy level of excitons produced in an emission layer, such that such excitons are not able to exist in the blocking layer. Specifically, the band gap of the blocking layer is at least as great as the difference in energy between the triplet state and the ground state of the host.
It is desirable for OLEDs to be fabricated using materials that provide electroluminescent emission in a relatively narrow band centered near selected spectral regions, which correspond to one of the three primary colors, red, green and blue so that they may be used as a colored layer in an OLED or SOLED. It is also desirable that such compounds be capable of being readily deposited as a thin layer using vacuum deposition techniques so that they may be readily incorporated into an OLED that is prepared entirely from vacuum-deposited organic materials.
Co-pending application U.S. Ser. No. 08/774,087, filed Dec. 23, 1996, now U.S. Pat. No. 6,048,630, is directed to OLEDs containing emitting compounds that produce a saturated red emission.
The present invention is directed to organic light emitting devices wherein the emissive layer comprises an emissive molecule, optionally with a host material (wherein the emissive molecule is present as a dopant in said host material), which molecule is adapted to luminesce when a voltage is applied across the heterostructure, wherein the emissive molecule is selected from the group of phosphorescent organometallic complexes. The emissive molecule may be further selected from the group of phosphorescent organometallic platinum, iridium or osmium complexes and may be still further selected from the group of phosphorescent cyclometallated platinum, iridium or osmium complexes. A specific example of the emissive molecule is fac tris(2-phenylpyridine) iridium, denoted (Ir(ppy)3) of formula 
[In this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line.]
The general arrangement of the layers is hole transporting layer, emissive layer, and electron transporting layer. For a hole conducting emissive layer, one may have an exciton blocking layer between the emissive layer and the electron transporting layer. For an electron conducting emissive layer, one may have an exciton blocking layer between the emissive layer and the hole transporting layer. The emissive layer may be equal to the hole transporting layer (in which case the exciton blocking layer is near or at the anode) or to the electron transporting layer (in which case the exciton blocking layer is near or at the cathode).
The emissive layer may be formed with a host material in which the emissive molecule resides as a guest or the emissive layer may be formed of the emissive molecule itself. In the former case, the host material may be a hole-transporting material selected from the group of substituted tri-aryl amines. The host material may be an electron-transporting material selected from the group of metal quinoxolates, oxadiazoles and triazoles. An example of a host material is 4,4xe2x80x2-N,Nxe2x80x2-dicarbazole-biphenyl (CBP), which has the formula: 
The emissive layer may also contain a polarization molecule, present as a dopant in said host material and having a dipole moment, that affects the wavelength of light emitted when said emissive dopant molecule luminesces.
A layer formed of an electron transporting material is used to transport electrons into the emissive layer comprising the emissive molecule and the (optional) host material. The electron transporting material may be an electron-transporting matrix selected from the group of metal quinoxolates, oxadiazoles and triazoles. An example of an electron transporting material is tris-(8-hydroxyquinoline) aluminum (Alq3).
A layer formed of a hole transporting material is used to transport holes into the emissive layer comprising the emissive molecule and the (optional) host material. An example of a hole transporting material is 4,4xe2x80x2-bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl [xe2x80x9cxcex1-NPDxe2x80x9d].
The use of an exciton blocking layer (xe2x80x9cbarrier layerxe2x80x9d) to confine excitons within the luminescent layer (xe2x80x9cluminescent zonexe2x80x9d) is greatly preferred. For a hole-transporting host, the blocking layer may be placed between the luminescent layer and the electron transport layer. An example of a material for such a barrier layer is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also called bathocuproine or BCP), which has the formula: 
For a situation with a blocking layer between a hole-conducting host and the electron transporting layer (as is the case in Example 2 below), one seeks the following characteristics, which are listed in order of relative importance.
1. The difference in energy between the LUMO and HOMO of the blocking layer is greater than the difference in energy between the triplet and ground state singlet of the host material.
2. Triplets in the host material are not quenched by the blocking layer.
3. The ionization potential (IP) of the blocking layer is greater than the ionization potential of the host. (Meaning that holes are held in the host.)
4. The energy level of the LUMO of the blocking layer and the energy level of the LUMO of the host are sufficiently close in energy such that there is less than 50% change in the overall conductivity of the device.
5. The blocking layer is as thin as possible subject to having a thickness of the layer that is sufficient to effectively block the transport of excitons from the emissive layer into the adjacent layer.
That is, to block excitons and holes, the ionization potential of the blocking layer should be greater than that of the HTL, while the electron affinity of the blocking layer should be approximately equal to that of the ETL to allow for facile transport of electrons.
[For a situation in which the emissive (xe2x80x9cemittingxe2x80x9d) molecule is used without a hole transporting host, the above rules for selection of the blocking layer are modified by replacement of the word xe2x80x9chostxe2x80x9d by xe2x80x9cemitting molecule.xe2x80x9d]
For the complementary situation with a blocking layer between a electron-conducting host and the hole-transporting layer one seeks characteristics (listed in order of importance):
1. The difference in energy between the LUMO and HOMO of the blocking layer is greater than the difference in energy between the triplet and ground state singlet of the host material.
2. Triplets in the host material are not quenched by the blocking layer.
3. The energy of the LUMO of the blocking layer is greater than the energy of the LUMO of the (electron-transporting) host. (Meaning that electrons are held in the host.)
4. The ionization potential of the blocking layer and the ionization potential of the host are such that holes are readily injected from the blocker into the host and there is less than a 50% change in the overall conductivity of the device.
5. The blocking layer is as thin as possible subject to having a thickness of the layer that is sufficient to effectively block the transport of excitons from the emissive layer into the adjacent layer.
[For a situation in which the emissive (xe2x80x9cemittingxe2x80x9d) molecule is used without an electron transporting host, the above rules for selection of the blocking layer are modified by replacement of the word xe2x80x9chostxe2x80x9d by xe2x80x9cemitting molecule.xe2x80x9d]
The present invention covers articles of manufacture comprising OLEDs comprising a new family of phosphorescent materials, which can be used as dopants in OLEDs, and methods of manufacturing the articles. These phosphorescent materials are cyclometallated platinum, iridium or osmium complexes, which provide electroluminiscent emission at a wavelength between 400 nm and 700 nm. The present invention is further directed to OLEDs that are capable of producing an emission that will appear blue, that will appear green, and that will appear red.
More specifically, OLEDs of the present invention comprise, for example, an emissive layer comprised of platinum (II) complexed with Bis[2-(2-phenyl)pyridinato-N,C2], Bis[2-(2xe2x80x2-thienyl)pyridinato-N,C3], and Bis[benzo(h)quinolinato-N,C]. The compound cis-Bis[2-(2xe2x80x2-thienyl)pyridinato-N,C3] Pt(II) gives a strong orange to yellow emission.
The invention is further directed to emissive layers wherein the emissive molecule is selected from the group of phosphorescent organometallic complexes, wherein the emissive molecule contains substituents selected from the class of electron donors and electron acceptors. The emissive molecule may be further selected from the group of phosphorescent organometallic platinum, iridium or osmium complexes and may be still further selected from the group of phosphorescent cyclometallated platinum, iridium or osmium complexes, wherein the organic molecule contains substituents selected from the class of electron donors and electron acceptors.
The invention is further directed to an organic light emitting device comprising a heterostructure for producing luminescence, wherein the emissive layer comprises a host material, an emissive molecule, present as a dopant in said host material, adapted to luminesce when a voltage is applied across the heterostructure, wherein the emissive molecule is selected from the group consisting of cyclometallated platinum, iridium or osmium complexes and wherein there is a polarization molecule, present as a dopant in the host material, which polarization molecule has a dipole moment and which polarization molecule alters the wavelength of the luminescent light emitted by the emissive dopant molecule. The polarization molecule may be an aromatic molecule substituted by electron donors and electron acceptors.
The present invention is directed to OLEDs, and a method of fabricating OLEDs, in which emission from the device is obtained via a phosphorescent decay process wherein the phosphorescent decay rate is rapid enough to meet the requirements of a display device. More specifically, the present invention is directed to OLEDs comprised of a material that is capable of receiving the energy from an exciton singlet or triplet state and emitting that energy as phosphorescent radiation.
The OLEDs of the present invention may be used in substantially any type of device which is comprised of an OLED, for example, in OLEDs that are incorporated into a larger display, a vehicle, a computer, a television, a printer, a large area wall, theater or stadium screen, a billboard or a sign.
The present invention is also directed to complexes of formula L Lxe2x80x2Lxe2x80x3 M, wherein L, Lxe2x80x2, and Lxe2x80x3 are distinct bidentate ligands and M is a metal of atomic number greater than 40 which forms an octahedral complex with the three bidentate ligands and is preferably a member of the third row (of the transition series of the periodic table) transition metals, most preferably Ir and Pt. Alternatively, M can be a member of the second row transition metals, or of the main group metals, such as Zr and Sb. Some of such organometallic complexes electroluminesce, with emission coming from the lowest energy ligand or MLCT state. Such electroluminescent compounds can be used in the emitter layer of organic light emitting diodes, for example, as dopants in a host layer of an emitter layer in organic light emitting diodes. This invention is further directed to organometallic complexes of formula L Lxe2x80x2Lxe2x80x3 M, wherein L, Lxe2x80x2, and Lxe2x80x3 are the same (represented by L3M) or different (represented by L Lxe2x80x2Lxe2x80x3 M), wherein L, Lxe2x80x2, and Lxe2x80x3 are bidentate, monoanionic ligands, wherein M is a metal which forms octahedral complexes, is preferably a member of the third row of transition metals, more preferably Ir or Pt, and wherein the coordinating atoms of the ligands comprise sp2 hybridized carbon and a heteroatom. The invention is further directed to compounds of formula L2MX, wherein L and X are distinct bidentate ligands, wherein X is a monoanionic bidentate ligand, wherein L coordinates to M via atoms of L comprising sp2 hybridized carbon and heteroatoms, and wherein M is a metal forming an octahedral complex, preferably iridium or platinum. It is generally expected that the ligand L participates more in the emission process than does X. The invention is directed to meridianal isomers of L3M wherein the heteroatoms (such as nitrogen) of two ligands L are in a trans configuration. In the embodiment in which M is coordinated with an sp2 hybridized carbon and a heteroatom of the ligand, it is preferred that the ring comprising the metal M, the SP2 hybridized carbon and the heteroatom contains 5 or 6 atoms. These compounds can serve as dopants in a host layer which functions as a emitter layer in organic light emitting diodes.
Furthermore, the present invention is directed to the use of complexes of transition metal species M with bidentate ligands L and X in compounds of formula L2MX in the emitter layer of organic light emitting diodes. A preferred embodiment is compounds of formula L2IrX, wherein L and X are distinct bidentate ligands, as dopants in a host layer functioning as an emitter layer in organic light emitting diodes.
The present invention is also directed to an improved synthesis of organometallic molecules which function as emitters in light emitting devices. These compounds of this invention can be made according to the reaction:
L2M(xcexc-Cl)2ML2+XHxe2x86x92L2MX+HCl
wherein L2M(xcexc-Cl)2ML2 is a chloride bridged dimer with L a bidentate ligand, and M a metal such as Ir; XH is a Bronsted acid which reacts with bridging chloride and serves to introduce a bidentate ligand X, where XH can be, for example, acetylacetone, 2-picolinic acid, or N-methylsalicyclanilide, and H represents hydrogen. The method involves combining the L2M(xcexc-Cl)2ML2 chloride bridged dimer with the XH entity. The resultant product of the form L2MX has approximate octahedral disposition of the bidentate ligands L, L, and X about M.
The resultant compounds of formula L2MX can be used as phosphorescent emitters in organic light emitting devices. For example, the compound wherein L=(2-phenylbenzothiazole), X=acetylacetonate, and M=Ir (the compound abbreviated as BTIr) when used as a dopant in 4,4xe2x80x2-N,Nxe2x80x2-dicarbazole-biphenyl (CBP) (at a level 12% by mass) to form an emitter layer in an OLED shows a quantum efficiency of 12%. For reference, the formula for CBP is: 
The synthetic process to make L2MX compounds of the present invention may be used advantageously in a situation in which L, by itself, is fluorescent but the resultant L2MX is phosphorescent. One specific example of this is where L=coumarin-6.
The synthetic process of the present invention facilitates the combination of L and X pairs of certain desirable characteristics. For example, the present invention is further directed to the appropriate selection of L and X to allow color tuning of the complex L2MX relative to L3M. For example, Ir(ppy)3 and (ppy)2Ir(acac) both give strong green emission with a xcexmax of 510 nm (ppy denotes phenyl pyridine). However, if the X ligand is formed from picolinic acid instead of from acetylacetone, there is a small blue shift of about 15 nm.
Furthermore, the present invention is also directed to a selection of X such that it has a certain HOMO level relative to the L3M complex so that carriers (holes or electrons) might be trapped on X (or on L) without a deterioration of emission quality. In this way, carriers (holes or electrons) which might otherwise contribute to deleterious oxidation or reduction of the phosphor would be impeded. The carrier that is remotely trapped could readily recombine with the opposite carrier either intramolecularly or with the carrier from an adjacent molecule.
The present invention, and its various embodiments, are discussed in more detail in the examples below. However, the embodiments may operate by different mechanisms. Without limitation and without limiting the scope of the invention, we discuss the different mechanisms by which various embodiments of the invention may operate.